Carbon calculator: Natural and unnatural methane emissions

Ezine

Published: Feb 15, 2015

Author: David Bradley

Channels: Chemometrics & Informatics

Emissions analysis

Measurements of the sporadic and perpetual methane emissions from natural gas facilities and processing plants reveals a huge variation in quantities entering the atmosphere across the United States, the implications for the atmospheric carbon burden are important. Moreover, the same pattern or worse might be reflected in data from other regions across the globe.

A collaboration between teams at Carnegie Mellon University, in Pittsburgh, Pennsylvania, Colorado State University, in Fort Collins, Colorado, Aerodyne Research Inc., in Billerica, Massachusetts and Fort Lewis College, in Durango, Colorado, led by mechanical engineer Anthony Marchese of CSU, has found that there are wide variations in the amount of methane being emitted at US natural gas gathering facilities and processing plants. The researchers base their conclusion on samples taken from 114 gathering stations and 16 processing plants across 13 states in what they refer to as the most comprehensive field study so far on this previously little-researched part of the natural gas industry.

Methane specifics

The team's data analysis reveals facility-level methane emissions ranging from less than 1 kilogram per hour to more than two thirds of a tonne per hour. The rate of loss of this important fuel gas is in some sites as low as 0.01 percent but greater than 10 percent elsewhere. The percentage is the ratio of methane wasted as a proportion of the total quantity flowing through a given facility. The researchers point out that the smaller facilities with the lower gas throughput are the ones that are least efficient in terms of having loss rates greater than 10 percent, but of course total waste from those facilities may be lower than from a facility that appears more efficient based on the percentages.

"This is an exciting study because it is, by far, the largest and most comprehensive data set ever collected on direct methane emissions from the gathering sector," explains Marchese. "The results point to the gathering sector likely being a notable source of emissions and identify areas where emission reductions can be achieved."

Two back to back papers appear in the latest issue of the American Chemical Society journal Environmental Science & Technology. The team is working on using the data to estimate total methane emissions from all gathering and processing facilities in the country can be obtained. The team anticipates being able to publish the results from that follow-on work later this year.

The team suggests that the significant variations they saw between facilities of similar size and type arises because of differences in inlet and outlet pressures and abnormal process conditions. Indeed, it is only a few sites that contribute to most of the methane emitted. Specifically, about one in three of the gathering facilities account for almost 80 percent of the total methane emissions recorded. One in six of the gathering facilities using with compression and/or dehydration were deemed to have abnormal process conditions that gave rise to greater than predicted methane emissions at liquid storage tanks. The team adds that, on average, emissions at these "abnormal" facilities were approximately 300 percent higher than similar but well tuned facilities. Additionally, the team's measurements suggest that the methane loss rates at processing plants are generally much lower than at gathering facilities with none of the processing plants having a loss rate higher than 0.6 percent.

Attribution

"The results of this study and the direct measurements obtained, suggest the majority of emissions can be attributed to a relatively small number of facilities," explains team member Allen Robinson of Carnegie Mellon who led the field measurements for the study. "That means we may be able reduce the overall methane emissions from the gathering and processing sectors by focusing on sources at these higher-emitting sites."

It is worth noting that methane, the primary component of natural gas, is a potent greenhouse gas, and natural gas and petroleum systems are the largest single source of man-made methane emissions in the US. The researchers explain that their comprehensive field study has given us the best methane emissions data to date for the gathering and processing sectors. "Prior to our study, most of the information we had on methane emissions from these facilities originated from studies in the early 1990s," Marchese adds.

"The next step in the research is to develop a national estimate of emissions for the two midstream sectors - transmission & storage and gathering & processing," Robinson told SpectroscopyNOW. "This involves combining the new emissions data we have collected with activity information (e.g. numbers of compressors and how they operate). After that we will be thinking about the data from the perspective opportunities and costs of reducing methane emissions from the natural gas system."